It is known in the continuous manufacture of rectangular foam slabs, in which a foamable polyurethane reaction mixture is freely foamed, to apply a gas-permeable overlay to the free surface of the rising foam during the foaming process. Such an overlay is intended to prevent the formation of a convexly curved surface on the foam slab during the blowing. The curvature is produced on the one hand by the sticking of the flanks of the slab or strand to the lateral boundary surfaces of the apparatus, and on the other hand by the formation of a solid skin on the top, which stretches arcuately. This undesirable cupping is supposed to be prevented by the known methods by having the overlay keep the surface of the slab flat.
From German "Offenlegungsschrift" No. 2,123,216, it is known to lay gas-permeable webs on the rising foam underneath a lattice-like or jalousie-like decking for the production of rectangular soft foam slabs. The gas-permeable web is intended to eliminate the danger that the decking might sink into the foam mixture.
The apparatus of "Offenlegungsschrift" No. 2,123,216, has, however, the disadvantage that the decking for the equalization of the foam surface is contaminated by the rising mixture. Therefore, German "Offenlegungsschrift" No. 2,165,902 provides that the gas-permeable web itself is in turn covered by an impermeable shielding web which is intended to prevent the foam mixture from wetting the superimposed equalization means.
The above-described, known methods have the disadvantage that the gas-permeable web is clogged at least in part by the rising foam mixture and thus the uniform removal of the evolving reaction gas, which emerges from the foam surface especially after the end of the foaming process, is not assured. Furthermore, the use of a gas-permeable web which is removed from the foam slab after it has set and then discarded adds undesirably to the cost of the process. The use of an additional, fluid-tight shielding web, which consists for example of paper, makes the process still more expensive and complicated.